1. Field of the Invention
The present invention relates to a method of displaying gray scales of a plasma display panel (PDP), and more particularly, relates to a method and apparatus for more accurately expressing gray scales.
2. Description of the Background
Generally, a plasma display device displays characters or images using a PDP, which generates plasma by gas discharge. The PDP may include hundreds of thousands to millions of pixels (discharge cells) arranged in a matrix. PDPs may be direct current (DC) types or alternating current (AC) types according to driving voltage waveform patterns and discharge cell structures.
In a general AC PDP, a single field (1 TV field) may be divided into a plurality of subfields. Each subfield is assigned a weight, and a total of the weights respectively assigned to the plurality of subfields represents gray scales. Examples of a method representing gray scales will be described hereinafter. Assume that a single field is divided into eight subfields SF1 to SF8 and weights assigned to the subfields are 1, 2, 4, 8, 16, 32, 64, and 128, respectively.
In this case, when a discharge cell represents gray scale 1, the discharge cell is turned on in the first subfield SF1 and is turned off in the remaining subfields SF2 to SF8. If a discharge cell represents gray scale 27, the discharge cell is turned on in the first, second, fourth, and fifth subfields SF1, SF2, SF4, and SF5 (1+2+8+16=27). Further, gray scale 255 is represented by turning on the discharge cell from the first subfield to the eighth subfield SF1 to SF8. The gray scale is represented by adding up weights assigned to the subfields having the discharge cells selected to be turned on.
Each subfield of the PDP may include a reset period, an address period, and a sustain period. The address period selects discharge cells to be turned on in a corresponding subfield, and the sustain period sustain-discharges the selected discharge cells during a period corresponding to a weight assigned to the corresponding subfield. Herein, the duration of the sustain period, in other words, the amount of light emitted due to the sustain-discharge during the sustain period, determines weight values. Substantially, the amount of light emission within a single subfield is the sum of the amount of light emitted due to the sustain discharge and an address discharge. For instance, in the case that the amount of light emitted due to the address discharge and the first occurrence of a sustain-discharge is set to be 2 (cd/m2) and the amount of light emitted due to a sustain pulse of one period is set to be 1.4, the amount of light emission for each gray scale is shown in FIG. 1.
Referring to FIG. 1, the amount of light emission for gray scale 7 and gray scale 8 are almost identical, and the amount of light emission for gray scale 15 is greater than the amount of light emission for gray scale 16. When representing gray scale 7, the address discharge occurs three times because the discharge cell is turned on in the first, second, and third subfields, whereas the address discharge occurs once for gray scale 8 because the discharge cell is turned on only in the fourth subfield to represent gray scale 8. However, according to the foregoing assumption, two emissions of the amount of light emitted from the address discharge (1.2) is almost identical to the amount of light emitted due to the sustain discharge (1.4), and therefore gray scales 7 and 8 are represented as almost the same. In like manner, the address discharge occurs four times for gray scale 15, whereas the address discharge occurs once for gray scale 16. Accordingly, the amount of light emission for gray scale 15 is greater than the amount of light emission for gray scale 16, thereby resulting in a reverse gray scale.
Accordingly, in a conventional subfield structure, a gray scale to be displayed on the screen may not always be represented as it is supposed to be when actually displayed on the screen.